The present invention relates generally to integrated power switches and more particularly to a method and system for providing accuracy for such switches in current limited applications.
Large integrated power MOS transistors are used to switch power from a source to a load in many circuits. For the protection of the switch itself and the load some form of current limiting technique is deployed. The aim of this current limit is not to let the load current to increase above a fixed or user adjusted value. All conventional circuits achieve this goal with a serious limitation. Above a certain input voltage in dead-short condition the output current unexpectedly increases to dangerous levels due to impact ionization inside the power transistor.
Accordingly, what is needed is a system and method that overcomes this problem. The system and method should be easy to implement, cost effective and adaptable to existing integrated circuit devices. The present invention addresses such a need.
An integrated power switch is disclosed. The power switch comprises a power transistor for providing an output current. The power transistor includes a grounded body. The power switch includes a sense transistor coupled to the power transistor. The sense transistor includes a floating body. The power switch further includes a resistor coupled to the floating body of the sense transistor. A value of the resistor is chosen such that the output current is regulated at a predetermined level.
Accordingly, a power switch in accordance with the present invention includes a power transistor which is implemented with grounded body and its companion sense transistor is implemented with floating body. Higher operating voltages can be achieved by turning on the parasitic transistor inside the sense transistor with a resistor connected from its body to ground. The impact ionization current of the power and sense transistors are included in the current limit loop control. The operating voltage and current range of an integrated power switch is extended by including the effect of impact ionization current in the current limit control loop.